Sheet metal workpieces of an alloy of suitable formability may be stretched and/or drawn against or over a forming tool surface at an elevated temperature to form a shape of complex deformation. For example, automotive body panels are being formed using sheet metal blanks of fine-grained aluminum alloy 5083.
In a high temperature sheet metal forming process, a formable metal material capable of high ductility under proper conditions is deformed against a surface of a forming tool. Fine-grain aluminum alloys have been known to achieve elongations of up to 1200% when heated to a temperature in the range of 400° C. to 550° C. and subjected to suitable strain rates. Similarly, fine grain titanium alloys have been known to achieve elongations of up to 1100% when heated to a temperature in the range of 815° C. to 1000° C.
Examples of a high temperature sheet metal forming processes include superplastic forming (SPF), quick plastic forming (QPF), and warm forming. SPF and QPF processes are examples of hot blow forming (also called hot stretch forming) processes used extensively with aluminum alloys in the manufacture of automobile components such as inner and outer lift gate panels, inner and outer door panels, deck lid panels, and the like. The sheet metal workpiece is positioned with one side lying close to the hot forming surface of a heated forming tool. The metal sheet is often preheated to its forming temperature so that both the workpiece and forming surface are at forming temperatures. A pressurized fluid, such as air is applied to the other side of the sheet forcing and stretching it into conformance with the tool surface, A common feature of these sheet metal parts is that they often require extensive localized deformation of the sheet metal to form a desired panel shape. In the case of automobile body panels, the parts may be formed in a single hot stretch forming stage or multiple forming steps may be used.
There is aggressive contact between the contacting side of the hot sheet metal and hot forming tool surface as the metal is deformed, and lubrication between the contacting surfaces is normally required. Sliding contact between the deforming metal workpiece and the forming tool often leads to problems associated with friction and adhesion. Adhesion may lead to galling patterns appearing on the finished workpiece surfaces, and forcible separation of the workpiece from the forming tool can distort the deformed metal workpiece beyond its allowable dimensions. Moreover, friction that occurs while the metal sheet is contacting the forming surface of the forming tool can lead to scratches, marks, and excessive roughness on the surface of the deformed metal workpiece, which may prevent the workpiece from meeting surface appearance requirements. Furthermore, this friction may affect the forming tool's forming surface, which may adversely affect all subsequently deformed metal workpieces by continuously reproducing undesirable abrasions.
Several lubricant coating materials have been developed to minimize friction and adhesion between the metal sheet and the forming tool. Boron nitride and graphite have been utilized for such purposes. Magnesium hydroxide [Mg(OH)2] and mixtures of Mg(OH)2 and boron nitride (BN) are also used. These materials have been used in the hot stretch forming of sheet metal blanks of AA5083 into body panels. Milk of magnesia may supply the requisite Mg(OH)2. MgO is also used as a precursor to Mg(OH)2.
Forming tool coatings have also been used. For example, a suitable coating may consist essentially of either a tungsten carbide cermet or a chromium carbide cermet having a particle size not more than 0.1 micrometers.
Still, there is a need for other lubricants for elevated temperature sheet metal forming, especially where light weight metal alloys such as aluminum alloys are formed.